7. Project Summary/Abstract Infections of the human genital tract by the obligate intracellular pathogen Chlamydia trachomatis are among the most highly reported sexually transmitted infections both in the U.S. and abroad. Asymptomatic chlamydial infections are highly prevalent and contribute to the frequency of undiagnosed and untreated patients. Untreated chlamydial infections can promote the development of serious sequelae in women, including pelvic inflammatory disease and tubal factor infertility. Despite these severe consequences, how Chlamydia manifest asymptomatically remains poorly understood. A growing body of evidence implicates an aberrant chlamydial growth phenotype, known as persistence, in abetting these chronic diseases. Varied and seemingly unrelated stimuli are known to facilitate persistence in vitro, yet it is unclear whether these various insults arrive at persistence through convergent or divergent pathways. Limiting the availability of iron to Chlamydia promotes persistence, but little is understood about how chlamydial species respond to iron limitation. The only identified iron-dependent transcriptional regulator in Chlamydia is represented by YtgR, the activity of which requires cleavage from an N-terminal permease domain, YtgC. Briefly limiting iron to C. trachomatis significantly induces the expression of the sole tryptophan (Trp) biosynthetic genes trpBA, indicating a previously undescribed iron- dependent mode of regulation controls Trp biosynthesis in C. trachomatis. YtgC possesses a unique codon motif of three sequential Trp residues (WWW), necessitating Trp availability for the translation of YtgC and thus YtgR. Intriguingly, starvation of Trp is one of the principal methods of inducing persistence as it represents a critical nutrient for Chlamydia. It is therefore possible that the loss of YtgR due to chronic Trp starvation would result in the dysregulation of the YtgR regulon, contributing to the induction of persistence. To investigate this hypothesis, we propose to utilize recent advances in chlamydial genetics to elucidate both the iron-dependent and Trp- dependent regulons of YtgR. First, we will define the iron-dependent YtgR regulon to understand the contribution of this regulator to the development of persistence induced via iron limitation. Secondly, we will elucidate the molecular mechanisms underpinning the iron-dependent regulation of trpBA. Finally, we will assess the role of the WWW motif in maintaining the YtgR regulon as a function of Trp availability. Successful completion of this research will provide the first mechanistic link between two separate stimulators of persistence, potentially offering key insights into the molecular basis for asymptomatic and chronic chlamydial infections.